I mean, even the latest demo from ps4 possibilities or any demo, you can see that the person is not real if there are eyes involved. E.g. if you only see the skin, you may be mistaken, but not if you see the eyes.

Why is it so difficult to achieve believable eyes in game development?

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This seems like a question more suitable for graphicdesign.stackexchange.com. You would probably get better answers there since this is basically a question relating to 3d graphics.
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GigggasFeb 21 '13 at 8:33

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Looks like a gamedev question to me. He asks about how it is done in live demos, not 3D Max pics. However "how it's made" usually considered offtopic here.
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Krom SternFeb 21 '13 at 8:39

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Are you really certain it is the eyes that are wrong, and not the facial musculature (i.e., facial expressions) in general? People are really good at pointing out "blank" expressions, but bad at pointing out why they are blank, and often blame the eyes which is rather vague. There seems to be a widely held belief that eyes somehow, "magically" express character and emotion. This, of course, doesn't bear any amount of scrutiny.
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JunuxxFeb 21 '13 at 10:30

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Even though you've selected an answer as "correct", I don't think there is a correct answer to this. The thing that's lacking from implementation to implementation may be different. I wouldn't think every attempt at making real eyes has the same flaw.
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Byte56♦Feb 21 '13 at 21:47

5 Answers
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I think it is because eyes are always wet. It is almost impossible to simulate wet surface with classic hardware reasterization techniques (I don't mean simulating water, but an object with very thin layer of water).

Hmm, seems a good point to me why it is so difficult. I have never think about it like that. Thanks
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DerfderFeb 21 '13 at 19:07

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-1: wet surfaces are really not that difficult nowadays. With physically-based shading models it is actually rather easy; you just set your specular parameters to those of water.
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Nathan ReedFeb 21 '13 at 23:42

@IvanKuckir Google it; there's tons of stuff on the web about physically-based shading. A good place to start is the SIGGRAPH 2012 course - the first two talks give a general introduction to the subject.
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Nathan ReedFeb 22 '13 at 3:53

Eyes are just balls. There's nothing expressive about a ball. What's expressive are the muscles around the eye, which not many games model. For example, think about the squint of anger, or the relaxed eyelids when you're bored.

I think we are much more sensible to the look of eyes than any other type of "surface" as we are acustomed to see them from a close distance and look at them alot.
Water or other surfaces may look differently depending on many factors, like water color, depth, lighting from above, lighting from under the water. We don't recognize errors there because it is hard to say if it looks right or not.

Additionaly eyes are very hard to render as they reflect/refract lights on multiple layers, have moving parts and are not perfect spheres.

Eyes are hard to do because of their complexity. They have a complex motion and a complex structure. This makes duplicating their aspect and motion hard to do in a "resource-friendly" manner.

Also, it might just not be that eyes are so badly done as compared to other stuff as it is more an issue of the fact that eyes are amongst the most studied parts of humans and what's around us. Maybe milk is not properlly done as well, heck, maybe it's worse than with eyes. Milk surface is tricky, there's lots and lots of sub-surface scattering and such. But since people tend to look more (and are more familiar with the aspect of) eyes than milk, it might just make so that eyes rendering is more criticized than milk rendering.

Also, at a supperficial glance, eyes seem easy. I mean they're just a shiny ball with some concentric cirles of various colors "painted" on, right :) ? This might lead the junior 3d artist into over-simplifying the task and making just that, a shiny ball with concentric textures that completly lacks, for example the subtle light sparkle that happends in every (living) eye.

Finally, eyes are small. Think XCOM Enemy Unknown (the 2012 version). 80% of the time you're in high-above-3rd-person view, you don't really see the eyes. But sometimes the camera zooms and changes angles so that you get a close-up portrait of the character. In such a setup (which is quite often), details which are not often seen from up-close (such as eyes) are intentionally left "un-detailed" so as to speed up development and conserve resources.

Best answer in my opinion. Would add the fact that the motion of the eyes, each alone and as a pair can be quite complex as well. In motion the eyes might look artificial but if you pause and look at the still image it might look more real. That is also true in several other aspects of animation, if the model is super good but the animations are incorrect we will notice it is "not human" as soon as it starts moving.
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RickardFeb 21 '13 at 23:24

Well eyes have a lot of reflection and refraction in them. That level of reflection would take up a lot of processing time. I suppose that developers would rather use that processing power to rather make the water more detailed in it's reflection. Also you're going to need relatively detailed textures for the eyes and considering it's such a small space, you're spending a lot of processing in a very small space.

If you're talking about the movement of eyes. I think some games have gotten it pretty good. Movement is also not that hard as you just have the eye ball look at an object (point an axis at the object) you want to look at. Having said that I have played some modern games, where not even this is done.